In recent years there has been a growing interest in coated piezoelectric crystals, not only as highly sensitive and selective detector of various air pollutants but also as simple, inexpensive and portable device. The high sensitivity and simple relationship between mass and frequency make the quartz crystal microbalance as an ideal tool for the study of adsorption and as a selective chemical sensor in many applications.
The principle of the detection is that the frequency of vibration of an oscillating crystal is decreased by the adsorption of a foreign material on its surface. A gaseous pollutant is selectively adsorbed by a coating on the crystal surface, thereby increasing the weight of the crystal and decreasing the frequency of vibration. The decrease in the frequency is proportional to the increase in weight due to the presence of gas adsorbed on the coating according to the following equation: ΔF=K·ΔC. Here, ΔF is the frequency change (Hz), K is a constant which refers to the basic frequency of the quartz plate, area coated, and a factor to convert the weight of injected gas (g) into concentration (ppm), and ΔC is concentration (ppm) of sample gas.
U.S. Pat. No. 3,164,004 teaches that a piezoelectric quartz crystal coated with a substrate selectively sensitive to changes in the atmospheric environment can serve as a detection device in fluid analyzers. In general, this discovery is based on the principle that the oscillation of a crystal, both in frequency and amplitude, is in part a function of its weight. The change in weight of a crystal coated with a substrate selectively sensitive to a particular contaminant when placed in an environment containing that contaminant is, in turn, at least partly a function of the concentration of the contaminant. Therefore, the measurement of the change in oscillation characteristics of a coated crystal sensitive to a particular contaminant upon exposure to a given atmosphere is a direct and highly sensitive measure of the presence and concentration of that contaminant. Variations of and improvements in this basic method are shown, inter alia, in the following publications U.S. Pat. No. 5,177,994; U.S. Pat. No. 5,817,921, and U.S. Pat. No. 6,085,576; Japanese Patents Nos. 1244335, and 5187986; European Patent No. 992768, and “Electronic Nose and Artificial neural Networks”, L. Moy and M. Collins, American Chemical Society, Anal. Chem., 1986, 58, pp. 3077–3084; “Piezoelectric Crystal Sensor for the Determination of Formaldehyde in Air”, Talanta, Vol. 38, No. 5, pp. 541–545, 1991; “Odor Sensing System Using Neural Network Pattern Recognition”, Toyosaka Moriiznmi and Takamichi Nakamoto, International Conference on Industrial Electronics, Control, Instrumentation and Automation, Nov. 9–13, 1992, Marriot Mission Valley, San Diego, USA.
A sensor has two equally important requirements: sensitivity and selectivity. There are two ways of achieving high selectivity and specificity towards xenobiotic (non-self) agents as we can learn from nature: (i) the immune system, in which a unique sensor (i.e. antibody) is being synthesized for any invader (i.e. antigen). This is a very complicated mechanism that involves a spontaneous constant synthesis of new molecules that are examined to fit the antigen; (ii) the olfactory system, in which a huge array of receptors are located in the nose in such a way that a molecule entering the nose interacts with some of the receptors; the brain then translates the pattern of the signals to an odor. In this case the odor can be a single molecule or a composition of several different molecules.
The combination of a number of sensors and a pattern recognition routine is known as an “electronic nose”. Using the combination of chemical sensors, which produce a fingerprint of the vapor or gas, the recognition algorithms can identify and/or quantify the analytes of interest. The electronic nose is capable of recognizing unknown chemical analytes, odors, and vapors. In practice, an electronic nose is presented with a substance such as an odor or vapor, and the sensor converts the input of the substance into a response, such as an electrical response. The response is then compared to known responses that have been stored previously. By comparing the unique chemical signature of an unknown substance to “signatures” of known substances, the unknown analyte can be determined. A variety of sensors can be used in electronic noses that respond to various classes of gases and odors.
A wide variety of commercial applications are available for electronic noses including, but not limited to, detection of explosives or drugs, environmental toxicology, biomedicine, such as microorganism classification or detection, material quality control, food and agricultural product monitoring, ambient air monitoring, employee protection, emissions control, and product quality testing. Referring to the detection of explosives, a number of laboratory techniques for the detection of explosives are known, using gas chromatography, mass spectrometry, ion mobility spectroscopy, NMR, plasma chromatography and visible chromatography. While some of these techniques are capable of ppb detection, the detection systems need elaborate techniques for operation, are usually not portable and simple, and are thus not useful for field use.